1. Field of the Invention
This invention relates to novel ester compounds which can take a stable thermotropic liquid crystal state and can be utilized as a liquid crystalline material useful for use in optoelectronics related elements using a liquid crystal and electrochemichromism such as display for liquid crystal television receiver, optical printer head, opto-Fourier transform element, light valve and the like, and liquid crystal compositions containing these compounds.
2. Related Art Statement
At the present liquid crystal compounds are applied to various apparatuses as a displaying material and put to practical use in watch, personal calculator, small size television receiver and the like. These apparatuses use a cell containing a liquid crystalline material composed mainly of nematic liquid crystal and adopt a display system called as TN type or STN type. In this case, the operation of the cell is based on a weak interaction (.DELTA..epsilon.E.sup.2 /2) between dielectric anisotropy .DELTA..epsilon. and electric field E, so that the cell has a drawback that the response time to the electric field is as slow as few tens m sec. Therefore, when the cell is applied to the television receiver, an active matrix system in which many switching elements are arranged at respective pixels is generally used as the cell driving system, which becomes an obstruct for realizing a large size display screen. However, a liquid crystal cell having a high-speed response property as .mu. sec order and a property not changing orientation of liquid crystal molecule even in the switching-off of the electric field (memory property) is made possible by appearance of ferroelectric liquid crystals representing 4-(4-n-decyloxybenzylideneamino) cinnamic acid-2-methylbutyl ester (DOBAMBC) synthesized by R. B. Meyer et al. in 1975 and a new displaying system using these liquid crystals as proposed by N. A. Clark (Applied Phys. Lett. 1980, 36, 899). When employing the displaying element with these materials, it is possible to realize a liquid crystal television receiver with a simple matrix system in which a multiplex driving mode is used without using the switching elements, which is fairly advantageous as compared with the active matrix system in view of productivity, cost, reliability, formation of large size display screen and the like.
Therefore, many ferroelectric liquid crystalline materials are synthesized and proposed up to the present. In order that these ferroelectric liquid crystalline materials are used as a displaying material, they are required to have some properties, among which it is fundamentally required to exhibit a smectic C phase and to have a large spontaneous polarization over a wide temperature range near room temperature and to be a chemical stability. In a greater part of the early ferroelectric liquid crystals, however, the spontaneous polarization was as small as not more than 10 nC/cm.sup.2, and they were chemically unstable because Schiff's base was existent in their molecules.
Recently, there is reported the appearance of large spontaneous polarization through chemically stable ester compounds. For instance, a compound having the following formula: ##STR3## forms a liquid crystal of chiral smectic C phase over a temperature range of 78.7.degree..about.103.3.degree. C. and of cholesteric phase over a temperature range of 103.3.degree..about.120.8.degree. C., and the spontaneous polarization at 83.degree. C. of this liquid crystal is 89 nC/cm.sup.2 (Japanese Patent laid open No. 61-43).
On the other hand, in order to decrease the temperature range exhibiting a chiral smectic C phase, there is reported a synthesis of compounds having two rings. For instance, a biphenyl compound having the following formula: ##STR4## exhibits a chiral smectic C. phase at 44.degree. C. in the cooling (Japanese Patent laid open No. 59-118644).
Furthermore, there is reported phenylpyrimidine derived compounds exhibiting a stable chiral smectic C phase near room temperature. For example, a compound having the following formula: ##STR5## forms a liquid crystal of chiral smectic C phase over a temperature range of 40.7.degree..about.82.8.degree. C. and of smectic A phase over a temperature range of 82.8.degree..about.89.1.degree. C.
However, said ester compounds have a drawback that the temperature range exhibiting a chiral smectic C phase is higher. Further, said biphenyl compounds have a monotropic and unstable chiral smectic C phase. Furthermore, as to said phenylpyrimidine derived compounds, it is presumed that the response time at 43.degree. C. of this liquid crystal is as slow as 1,500 .mu.s and the spontaneous polarization is fairly small.
That is, in a liquid crystalline material for displaying apparatuses required to have a high-speed response property and the like, it is required to have a large spontaneous polarization and a low viscosity and to exhibit a chiral smectic C phase over a wide temperature, but it is actual circumstances that there is not yet developed a liquid crystalline material sufficiently satisfying their physical properties.
Recently, however, there are reported many compounds having a fluorine atom or a trifluoromethyl group on an asymmetric carbon in their molecule. For instance, Heppke et al. reported that a compound having a fluorine atom on an asymmetric carbon in its molecule and having the following formula: ##STR6## exhibits a chiral smectic C phase and a spontaneous polarization of not less than 400 nC/cm.sup.2 (Proceedings of the 12th International Conference on Liquid Crystal).
In addition, there is reported a compound having a trifluoromethyl group on an asymmetric carbon in its molecule and having the following formula: ##STR7## exhibits a chiral smectic C phase over a temperature range of 115.5.degree..about.93.1.degree. C. and a spontaneous polarization of 166 nC/cm.sup.2 (Japanese Patent laid open No. 64-3154).
There is anothr proposition on a derivative of 2-fluoro-2-methylbutyric acid having the following formula: ##STR8## However, there is no description of any properties of the compound (Japanese Patent laid open No. 1-501394).